Just an editorial point:
I find the description of compound steps in 2.1 rather unclear. What
are the contained steps of a p:choose? Are p:when and p:otherwise
steps, or just syntax? If the former, then 2.1 implies that they form
a subpipeline, when in fact each of them is a subpipeline. If the
latter, then p:choose doesn't have any contained steps, because the
steps don't "occur directly" within p:choose.
-- Richard